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(#) Passing an expression which always returns `Unit` as a key argument

!!! WARNING: Passing an expression which always returns `Unit` as a key argument
   This is a warning.

Id
:   `OpaqueUnitKey`
Summary
:   Passing an expression which always returns `Unit` as a key argument
Severity
:   Warning
Category
:   Correctness
Platform
:   Any
Vendor
:   Jetpack Compose
Identifier
:   androidx.compose.runtime
Feedback
:   https://issuetracker.google.com/issues/new?component=612128
Min
:   Lint 8.0 and 8.1
Compiled
:   Lint 8.0 and 8.1
Artifact
:   [androidx.compose.runtime:runtime-android](androidx_compose_runtime_runtime-android.md.html)
Since
:   1.5.0
Affects
:   Kotlin and Java files and test sources
Editing
:   This check runs on the fly in the IDE editor
Implementation
:   [Source Code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/runtime/runtime-lint/src/main/java/androidx/compose/runtime/lint/OpaqueUnitKeyDetector.kt)
Tests
:   [Source Code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/runtime/runtime-lint/src/test/java/androidx/compose/runtime/lint/OpaqueUnitKeyDetectorTest.kt)
Copyright Year
:   2023

Certain Compose functions including `remember`, `LaunchedEffect`, and
`DisposableEffect` declare (and sometimes require) one or more key
parameters. When a key parameter changes, it is a signal that the
previous invocation is now invalid. In certain cases, it may be required
to pass `Unit` as a key to one of these functions, indicating that the
invocation never becomes invalid. Using `Unit` as a key should be done
infrequently, and should always be done explicitly by passing the `Unit`
literal. This inspection checks for invocations where `Unit` is being
passed as a key argument in any form other than the `Unit` literal. This
is usually done by mistake, and can harm readability. If a Unit
expression is being passed as a key, it is always equivalent to move the
expression before the function invocation and pass the `Unit` literal
instead.

!!! Tip
   This lint check has an associated quickfix available in the IDE.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/test/test.kt:10:Warning: Implicitly passing Unit as argument to key1
[OpaqueUnitKey]
    val x = remember(unitProperty) { listOf(1, 2, 3) }
                     ------------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`src/test/test.kt`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers
package test

import androidx.compose.runtime.*

val unitProperty = Unit

@Composable
fun Test() {
    val x = remember(unitProperty) { listOf(1, 2, 3) }
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/runtime/runtime-lint/src/test/java/androidx/compose/runtime/lint/OpaqueUnitKeyDetectorTest.kt)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `OpaqueUnitKeyDetector.remember_withUnitPropertyRead_reportsError`.
To report a problem with this extracted sample, visit
https://issuetracker.google.com/issues/new?component=612128.

(##) Including

!!!
   This is not a built-in check. To include it, add the below dependency
   to your project.

```
// build.gradle.kts
implementation("androidx.compose.runtime:runtime-android:1.8.0-alpha07")

// build.gradle
implementation 'androidx.compose.runtime:runtime-android:1.8.0-alpha07'

// build.gradle.kts with version catalogs:
implementation(libs.runtime.android)

# libs.versions.toml
[versions]
runtime-android = "1.8.0-alpha07"
[libraries]
# For clarity and text wrapping purposes the following declaration is
# shown split up across lines, but in TOML it needs to be on a single
# line (see https://github.com/toml-lang/toml/issues/516) so adjust
# when pasting into libs.versions.toml:
runtime-android = {
    module = "androidx.compose.runtime:runtime-android",
    version.ref = "runtime-android"
}
```

1.8.0-alpha07 is the version this documentation was generated from;
there may be newer versions available.

NOTE: These lint checks are **also** made available separate from the main library.
You can also use `androidx.compose.runtime:runtime-lint:1.8.0-alpha07`.


[Additional details about androidx.compose.runtime:runtime-android](androidx_compose_runtime_runtime-android.md.html).
(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("OpaqueUnitKey")
  fun method() {
     remember(...)
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("OpaqueUnitKey")
  void method() {
     remember(...);
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection OpaqueUnitKey
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="OpaqueUnitKey" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'OpaqueUnitKey'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore OpaqueUnitKey ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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